A light emitting element is a element which emits light by applying an electric field. As to its light emission mechanism, it is said that by applying a voltage to electrodes, and interposing an organic compound layer therebetween, electrons which are injected from a cathode and holes which are injected from an anode, are recombined at a light emission center in an organic compound layer to form molecules which are in an excitation state (hereinafter, inscribed as “molecular exciter”), and the molecular exciter discharges energy to emit light on the occasion of returning to a ground state.
As types of molecular exciters that are organic compound form, a singlet excitation state and a triplet excitation state are possible, and in this specification, included are both cases in which either excitation state contributes to light emission.
In such light emitting element, normally, an organic compound layer is formed by a thin film with the thickness even less than 1 μm. Also, since the light emitting element is an element of a self-light emission type in which the organic compound layer itself emits light, there is also no necessity of a back light as used in a conventional liquid crystal display. Therefore, it is a big advantage that it is possible to fabricate a light emitting element of extremely a thin shape and lightness in weight.
Also, for example, in an organic compound layer of approximately 100–200 nm, time from carrier injection until recombination is approximately tens of nano seconds, considering carrier mobility of the organic compound layer. Light emission is realized with time within a micro second order, even if a process from recombination of carrier until light emission is included. Therefore, it is also one feature that a response speed is very fast.
Further, since the light emitting element is a light emitting element of a carrier injection type, driving by a direct-current voltage is possible, and generation of noise is reduced. With regard to a drive voltage, firstly, the organic compound layer is made to be a uniform ultra thin film with a thickness of approximately 100 nm. An electrode material is selected which lessens a carrier injection barrier to the organic compound layer, and furthermore, a single heterostructure (2 layer structure) is introduced, and thereby, achieved sufficient luminance of 100 cd/m2 at 5.5V. (See C. W. Tang et al., Applied Physics Letters, Vol. 51, No. 12, 913–915 (1987)).
From characteristics such as a thin shape and lightness in weight/high speed response/direct-current low voltage drive etc., the light emitting element is noticed as a next-generation flat panel display element. Also, since it is of a self-light emission type and a viewing field angle is wide, its visibility is relatively good, and it is considered to be effective as a element which can be used for a display screen of a portable equipment.
In the meantime, as a big problem of such light emitting element, reliability of an element is pointed out. Among reliability, in particular, deterioration over time of luminance is significant, and big improvement is necessary.
The deterioration over time of luminance is considered to be basically a phenomenon derived from a material which is used, but it is possible to lengthen a half-life period of luminance by a element structure and a driving method. For example, there is such an example that, as a hole injection layer, copper phthalocyanine (hereinafter, inscribed as “CuPc”) is inserted, and further, drive is carried out by an alternate current (constant current in case of forward bias, and constant voltage in case of reverse bias) of a rectangular wave but not by a direct current, the half-life period of luminance was largely improved. (See S. A. Van Slyke et al., Applied Physics Letters, Vol. 69, No. 15, 2160–2162 (1996)).
In Van Slyke et al., lengthening a luminance half-life period up to 4000 hours at initial luminance 510 cd/m2 was successfully attained. As its cause, cited are elimination of accumulation of space electrification by alternate-current drive, goodness of heat resistance of N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidene (hereinafter, inscribed as “NPB”) which is a hole transport layer, and that CuPc, which is a hole injection layer, has an excellent hole injection property.
Also, if surface treatment is applied to indium-tin-oxide (hereinafter, inscribed as “ITO”) which is used for an anode, a contact angle with water becomes almost 0°. A contact angle of NPB with water is approximately 70°–80°, and it is understandable that a difference of surface energies between ITO and NPB is very large. On this account, when a film of NPB is formed directly on ITO, NPB is easily crystallized, and deteriorated faster as an element. Inserting CuPc into a boundary face of ITO and NPB as a hole injection layer to suppress crystallization of NPB is also a cause which lengthened reliability.
As described above, by using CuPc for a hole injection layer, reliability of a light emitting element is improved, but it cannot be said that reliability is sufficient. As one of its reasons, it is pointed out that a film forming property is bad, and it is hard to prepare a uniform thin film.